1. Field of the Invention
The present invention is generally related to hydrophone devices for detecting sound or vibrational waves in a fluid medium and more specifically to a hydrophone device that includes improvements in casing design, improved transducer or vibration sensor design, and improved electronic signal processing, all of which contribute to substantially improved, stronger, and more usable output signals indicative of the actual vibrational waves detected with less detrimental noise and other signal degradation factors.
2. Description of the Prior Art
Hydrophones are essentially transducer devices that detect and convert sound or vibrational waves, i.e., series of compressions and rarefactions, in a fluid medium to readable electrical signals that are indicative of the sound or vibrational waves in the medium. Hydrophones can be used for a variety of purposes, including, for example, seismic exploration operations, sonar receivers, and the like.
In seismic exploration operations, such as for locating or analyzing geological features in the earth's crust, some means, such as an explosion, can be used to generate vibration waves in the earth, and the vibration waves reflected by the various geostructures can be detected by hydrophones placed at or near the earth's surface. In land-based seismic exploration operations, the hydrophones can be actually placed in holes bored into the earth and containing water or drilling fluid, which actually transmits the reflected vibration waves from the earth to the hydrophone transducer. In ocean-based seismic exploration operations, the hydrophone transducer can simply be placed in the sea water, which transmits the reflected vibration waves from the earth to the hydrophone transducer.
In active sonar applications, sound waves can be generated in the sea water by any sound-making device, and the sea water then transmits the sound waves to nearby objects and transmits sound waves reflected by the objects directly back to a hydrophone transducer placed in the sea water. Passive sonar operates in a similar manner with a hydrophone placed in the sea water, except the sea water merely transmits sound waves generated by nearby objects to the hydrophone transducer, so that the hydrophone acts as a passive listening device.
Signal quality or, more precisely, the lack of good signal quality generated by the hydrophone, is a common, pervasive problem in both seismic and sonar applications, as well as in many other hydrophone applications. There are many causes of such typical low quality signal generations by hydrophones. For example, in seismic operations, the earth has a well-known inherent filtering effect, which tends to attenuate signals, particularly in higher frequency ranges. Also, the signals are typically very weak and do not always have sharp definition characteristics, yet they usually have to be transmitted by wire many hundreds of feet to data collection points. Extraneous electromagnetic interference and other noises become a real problem with low amplitude signals being transmitted over such long transmission wires. Also, background noises and vibrations in both sonar and seismic operations can interfere and drown out the significant signals. As a result, signal-to-noise ratio at the data collection point is usually very low.
Further, typical hydrophone transducers utilize piezoelectric devices to convert physical vibrations to electrical signals. Such piezoelectric hydrophones are notoriously sensitive to extraneous electromagnetic interference, which is becoming more of a problem as geophysical acquisition in the industry is moving toward desiring more sensitive and higher resolution recordings of geophysical data.
Also, the ceramic piezoelectric crystals typically used in hydrophones can be over-flexed and damaged, such as possibly caused by exposure to a high pressure or by jarring or mishandling the hydrophone. This problem has been controlled in contemporary hydrophones by mounting the ceramic piezoelectric crystal on a thicker metal carrier plate. Unfortunately, however, the thicker the carrier plate is made for more protection of the ceramic crystal, the less sensitive the ceramic crystal becomes to sound or vibration waves. Thus, thicker support or carrier plates in the conventional mounting practice, while providing more physical protection for the ceramic crystal, thus pressure resistance of the hydrophone, also sacrifices sensitivity and resolution of geophysical signal data.
Amplifiers at the hydrophone location unfortunately do not solve these problems, since they amplify all components of the signal generated by the same degree, including environmental and circuit noise levels. A substantial improvement was made by my non-linear seismic line amplifier described in U.S. Pat. No. 4,706,226, which conditions the signal to counteract natural attenuation by the earth, provides an output signal having a substantially flat frequency response for a seismic impulse, and increases the signal-to-noise ratio. However, there are non-linear hydrophones available, and a vastly more significant advance would be represented by a hydrophone that produces a superior signal with better resolution and greater signal-to-noise ratio before amplification and conditioning.
Other problems have also persisted in the manufacture and use of hydrophones, particularly in geophysical explorations. For example, seismic crews usually try to monitor the sensor connection to their recording equipment with a simple ohm meter arrangement. High output impedances of some prior art hydrophones require shunt resistors across the output leads with resistances beyond the ranges of the line monitoring devices of the recording equipment, such as in the range of 100K. Thus, much lower output impedance of the hydrophone is desired, preferably such that a shunt resistor in the range of only about 10K can be used in monitoring the connection to the recording equipment.
Also, it is common in the seismic exploration industry for hydrophones to be disposable, i.e., used only once. Many contemporary fluid-coupled and pressure sensors utilize fluids such as naphtha, trichloroethylene, or automotive transmission fluid as the transmitting medium of a fluid couple, all of which not only have to be degassed to remove air bubbles to maintain sensitivity, but are also potentially damaging to environmentally sensitive areas, particularly when they get blown apart by the vibration-inducing explosions commonly used. Therefore, another substantial improvement would be to have a low power, battery operated hydrophone that can be reused indefinitely and fluid-coupling devices that are not only more sensitive, but which also do not have to be degassed and are environmentally safe.